As a memory system used in a computer system and the like, an SSD (Solid State Drive) that installs memory chips provided with NAND type memory cell arrays is drawing attention. The SSD has advantages such as fast speed, light weight and the like compared to a magnetic disk device.
The SSD internally performs lookahead to improve read performance upon a sequential read. The sequential read is an access pattern that reads sequentially in an arrangement order of logical addresses. A target of the lookahead is data with a predetermined size (lookahead length) with a logical address that is continued from a storage position of data of which read has been requested. If the lookahead length is too small, the read performance is deteriorated. If the lookahead length is too large, a delay amount (penalty) generated upon a prediction failure, that is, when data different from the data that had been looked ahead is requested to be read becomes large.